Which scope rules apply to variables defined within nested Pug mixins?

Question

Pug allows the creation of reusable components through mixins, which can be nested to build complex component hierarchies. While arguments passed to a mixin are locally scoped to that specific block, the behavior of variables declared inside a nested mixin relative to its parent mixin requires clarification.

The goal is to determine if a variable initialized within a child mixin can be accessed by the parent mixin after the child call completes, or if the local scope is strictly destroyed upon the child mixin's termination.

Does a variable declared inside a nested mixin leak into the parent mixin's scope?
  What is the precise behavior of variable shadowing when a parent and child mixin use the same variable name?

Tasadduq Burney · Answer

Direct answer

A variable defined inside a nested Pug mixin is scoped only to that child mixin and any inner blocks; it is destroyed when the child mixin call ends and is not accessible to the parent mixin.

Explanation

The lexical scope of a mixin is created at invocation time. Variables declared within the child mixin exist in that new scope and are removed after the mixin returns. If a parent and child mixin use the same name, the child’s declaration shadows the parent within its own scope, while the parent’s original value remains unchanged outside the child call.

Confirmed facts

Variables are local to the child mixin and its nested blocks.
After the child mixin finishes, its variables are destroyed.
Same‑named variables are shadowed; the parent’s variable remains unchanged after the call.
Assigning to an undeclared variable that exists in the parent will modify the parent’s variable, causing side effects.

Verification steps

Create a Pug file with a parent mixin that sets <code>var a = 1</code>, calls a child mixin that sets <code>var a = 2</code>, outputs <code>a</code> before the call, inside the child, and after the call.
Compile the template with <code>pug -o out template.pug</code> and inspect the generated HTML; <code>a</code> should show 1 before and after, and 2 only within the child block.
Render the compiled template in Node.js using <code>pug.renderFile</code> and log the HTML; verify that the parent’s variable value remains unchanged after the child call.

Missing diagnostic detail: please specify the Pug version you are using (e.g., <2.0) because older releases may have different scoping semantics.

Which scope rules apply to variables defined within nested Pug mixins?

1 answer

Direct answer

Explanation

Confirmed facts

Verification steps

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